This invention relates to the selective hydrogenation of alpha-methyl-styrene (AMS) to cumene, and more particularly to the selective hydrogenation of alpha-methyl-styrene to cumene using a combination of a nickel catalyst and a noble-metal catalyst to achieve an optimal combination of conversion and selectivity at a lower overall catalyst volume and cost.
The production of phenol from cumene by the KBR Phenol Process has been studied for more than 50 years. Typically, cumene can be oxidized with air to produce cumene hydroperoxide (CHP), which can be concentrated and cleaved to produce phenol and acetone in the presence of an acid catalyst using, for example, the KBR Advanced Cleavage System. The catalyst can be removed, and the mixture can be fractionated to produce high-purity products, principally phenol and acetone. The fractionation train can be designed to separate alpha-methyl-styrene (AMS) as co-product, or the AMS can be hydrogenated to cumene for recycle to the oxidation step. Representative patents disclosing the production of phenol from cumene via cleavage of the hydroperoxide include U.S. Pat. No. 3,290,384 to Largman, U.S. Pat. No. 3,646,235 to Little and U.S. Pat. No. 5,430,200 to Hood.
AMS is an inevitable by-product of the cumene-to-phenol process. AMS precursors can be formed in the cumene oxidation step, and converted to AMS during the subsequent cleavage step. AMS is generally viewed as an undesired reaction by-product due to difficulty in removing AMS from both the phenol in the rectification step and from the recycled cumene stream, and due to a relatively small market for AMS. When market conditions dictate, AMS can be separated, purified and exported for sale. AMS produced in the synthesis of phenol can also be hydrogenated to cumene and recycled to the oxidation step to enhance the overall yield of the phenol process.
Hydrogenation of AMS has previously been disclosed using noble-metal catalysts, notably palladium (Pd) on alumina (Al2O3) or carbon support, where the catalyst can be used in a fixed bed system in the liquid phase, or in a trickle bed system with both vapor and liquid phases. In U.S. Pat. No. 3,646,235, Little et al. disclose the use of nickel, platinum, palladium, cobalt, chromium oxide and mixed metal catalysts for the hydrogenation of AMS. A palladium catalyst having a metal content of between 1 and 5% is noted as being preferred, for use at temperatures of 24°-120° C. (75°-248° F.) and pressures of 0.17-0.86 MPa (25-125 psia), preferably at temperatures of 24°-50° C. (75°-122° F.) and pressures of 0.17-0.45 MPa (25-65 psia).
In U.S. Pat. No. 4,822,936, Maurer et al. disclose selective hydrogenation of phenylacetylene, in the presence of styrene, with a copper catalyst supported on gamma alumina. The use of palladium catalysts is discussed, but is discouraged due to the large excess of hydrogen required, often resulting in the hydrogenation of styrene.
In U.S. Pat. No. 5,064,507, O'Donnell et al. disclose a process for the purification of a crude phenol stream having between 0.5 and 10 weight percent AMS present by separating AMS from the crude phenol product using a steam distillation process.
In U.S. Pat. No. 5,905,178, Hildreth discloses the removal of AMS from cumene by selective hydrogenation of the side chain in a distillation column reactor. The process includes the step of contacting the cumene/AMS feed stream with hydrogen and catalyst material, with the catalyst specified as preferably being palladium oxide on an appropriate support. The reaction feed is given as 18 weight percent AMS and 82 weight percent cumene, with the catalyst zone being maintained at a pressure and temperature of 0.21 MPa (30 psia) and 174° C. (345° F.), respectively.
Hydrogenation of AMS has previously been disclosed using Raney nickel catalysts in a slurry process, but the process has been largely replaced with the fixed bed process due to loss of aromatics. Although the slurry process is effective, it requires two distillation towers and associated equipment, as well as energy in the form of cooling and pressure. Additionally, the Raney nickel-based catalysts have drawbacks, including producing undesired side products due to over-hydrogenation, and a need for frequent addition of fresh catalyst.
Noble metal hydrogenation catalysts typically can exhibit high conversion yields and selectivity, as well as long catalyst life with little need for regeneration. However, noble metal catalysts are generally substantially more expensive than typical nickel catalysts.
The present invention can provide a mixed catalyst system for the hydrogenation of AMS to cumene that takes advantage of the low cost and high activity of the nickel catalyst and the high selectivity and long catalyst life of the noble metal catalyst.